1. Field of Invention
The present invention relates generally to acoustic attenuation and vibration damping materials. More specifically, the invention provides a two-ply or greater composite acoustic attenuation and vibration damping material intended to be placed between acoustic and/or vibratory energy sources and acoustic and/or vibration protected areas.
2. Description of the Related Art
Within the field of noise control, absorptive techniques are typically utilized to prevent or reduce air-borne acoustic energy from reaching a receiving site. Similarly, vibration-damping techniques are usually applied in close contact with the vibrating structure to prevent or reduce air-borne or structure-borne energy from propagating to the protected area. Both techniques utilize internal damping of impinging acoustic energy as an important means of reducing energy levels and therefore share basic principles. A general review of the art in this area is available from “Material Damping and Slip Damping” by L. E. Goodman (Shock & Vibration Handbook (3rd ed.), Cyril M. Harris (ed.), 1987) and from “Sound-Absorptive Materials” by Ron Moulder (Handbook of Acoustical Measurements and Noise Control (3rd ed.), Cyril M. Harris (ed.), 1991), but a brief overview follows.
Currently available materials capable of absorbing unwanted acoustic energy (i.e., noise) are most effective at frequencies above 500 Hz. Noise attenuation rapidly worsens as lower frequencies are encountered with the result being that few material manufacturers even report attenuation values below 125 Hz.
Most sound absorptive materials, such as foams, felts, etc., are highly porous in structure with the pores intercommunicating throughout the material. The pores may be formed by interconnected solid bubbles or by interstices between small granules. The pores may also be inherent in naturally porous fibrous materials, such as fiberglass. The amplitude of sound waves entering the porous material is reduced through friction between the air molecules and the surfaces of the pores. These materials tend to be light in weight and most effective at shorter wavelengths (i.e., higher frequencies). Unless these porous materials form part of a layered, or constrained, composite with a denser, less porous material, their structural strength is limited.
In order to attenuate lower frequencies, absorptive materials are usually combined with a rigid material with an air space separating the two materials. The amount of low frequency attenuation is directly related to the size of this air space. This approach of combining a sound absorptive material with a rigid material and a separating air space increases both the overall weight and thickness of the resulting sound attenuating structure and therefore may not be feasible in a given application. A significant problem with this approach is the fact that many structures must be load bearing as well as sound absorbing, necessitating the inclusion of solid members between rigid materials. These solid members often provide a very good conduit for acoustic energy, thereby partially defeating the structures' sound attenuating properties.
Another approach embodies the “mass law” which applies to a relatively thin, homogeneous, single layer panel. The mass law states that the loss of energy as it transits a barrier is, over a wide frequency range, a function of the surface density of the barrier material and the frequency in question. In general, this transmission loss increases by 6 decibels (dB) for each octave increase in frequency and for each doubling of the mass of the material. Thus, increasing the mass of the material through increases in thickness or density can improve the acoustic barrier for all frequencies including those in the lower portion of the spectrum. This gain in transmission loss is at the cost of added barrier weight.
Materials utilized specifically for vibration damping follow many of the same rules as those in the absorptive class but are, as a general rule, optimized for attenuating the lower frequencies. As a result, many of these materials have higher density and thickness, tending to depend more on the internal damping of energy penetrating the material than upon the “capture” of acoustic energy by way of a porous architecture.
Absorbing or damping unwanted acoustic or vibrational energy involves converting acoustic energy into another form, usually heat. Heat, acoustic or vibrational energies are closely related. At the molecular level, the primary distinction between heat energy and acoustic or vibrational energy lies in the vector direction of molecular displacements. Acoustic and vibrational energy are characterized by molecular displacements with vector directions that are highly correlated, with large numbers of molecules displacing at the same time and in the same direction. Heat in a medium may well have similar or more energy than propagating acoustic or vibrational energy, but the motion of the molecules is in random directions with the mean molecular displacement at any given location being near zero. Thus, to dissipate acoustic or vibrational energy as heat involves mechanisms that de-correlate molecular movements into random directions.
Several techniques are available for de-correlating molecular movements into random directions. For example, Cushman, et al. (U.S. Pat. No. 5,400,296, incorporated by reference in its entirety herein) teaches the use of two or more species of particles with differing characteristic acoustic impedances embedded in a matrix material. Within the matrix material reflections at boundaries with higher impedance particles are in phase, and reflections at boundaries with lower impedance particles are out of phase. Reflections with different phase relationships at or near the same locale increase the probability of phase cancellations. Phase cancellations de-correlate molecular movements into random directions. However, overdriving an impedance mismatch can result in harmonic distortions that reduce or negate the attenuating properties of the material.
A second approach to de-correlating molecular movements involves the careful choice of matrix materials that exhibit a high degree of internal hysteresis. Propagating acoustic or vibrational energy may boost a particular molecule into a higher energy level, thus subtracting that energy from propagating energy, where the molecule remains for some time before randomly returning to its original energy level. For a discussion of this effect, see Hartmann and Jarzynski, “Ultrasonic hysteresis absorption in polymers,” J. Appl. Phys., Vol. 43, No. 11, November 1972, 4304-4312.
A third potential method for redirecting the molecular movements of acoustic or vibrational energy is to convert this energy into electricity using the piezoelectric effect, and to dissipate it as heat through resistive heating.
In addition to the various techniques for increasing acoustic absorption or vibration damping within a material, the shape of a material conducting acoustic or vibratory energy can be made to redirect acoustic energy in harmless directions or to promote viscous damping at an interface. Porous outer layers can be very advantageous. They may promote viscous damping within the interfacing medium, provide a larger surface area with the interfacing medium, and may act as phase shifters by exploiting the fact that the speed of sound in solid materials is much higher than in a gas.
It can be shown experimentally that thin panel sections with very good barrier capability are possible using the techniques described in Cushman, et al (U.S. Pat. No. 5,400,296). However, these panels are not immune to the laws of mechanics and when thin panel sections are attempted, the entire panel will simply follow Newton's well known relationship, F=ma. That is, the entire panel will move over in response to a pressure wave and act as a diaphragm on the opposite surface, thus re-creating the original pressure wave. Very little energy will enter the material where it may be dissipated. The only effective ways to prevent movement of thin sections in response to acoustic pressure are to a) increase the mass of the panel, b) to design the structure to optimize the stiffness of the panel against its support, and c) to reduce the resistance of the panel to incoming pressure waves by making it discontinuous. In many applications, increasing the mass of a barrier structure is not desirable, but increasing the stiffness is acceptable as is decreasing the resistance of the panel by making it discontinuous. A discontinuous panel is a good absorber but is not a good barrier. It may, however, be attached to a barrier panel and the combination provide benefits that neither can provide alone.